drm/amd/display: Drop pixel_clock_mhz

The pixel_clock_mhz property is populated in amdgpu_dm when Freesync is setup,
but it is not used anywhere in amdgpu_dm. Remove the dead code.

Cc: chiahsuan.chung@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>